1. Field of the Invention
The present invention relates, in general, to a device that allows complete repositioning of two members relative to one another. More specifically, this invention relates to the method for using an improved orthopaedic external fixator including a mechanism that allows two bone elements or portions to be fixed relative to one another while allowing complete repositioning of the two bone elements or portions relative to one another.
2. Background Art
It is often necessary to realign, reposition and/or securely hold two elements relative to one another. For example, in the practice of medicine, bone fragments and the like must sometimes be aligned or realigned and repositioned to restore boney continuity and skeletal function, etc. At times this may be accomplished by sudden maneuver, usually followed by skeletal stabilization with cast, plate and screws, intramedullary devices, or external skeletal fixators.
A bone fragment can be moved, in general, from its original position as in a nonunion or malunion or from its intended position as in congenital deformities along six separate axes, a combination of three orthogonal translational axes (e.g., typical "X," "Y" and "Z" axes) and three orthogonal rotational axes (e.g., rotation about such typical "X," "Y" and "Z" axes).
Certain boney skeletal injuries or conditions are sometimes treated with an external device that is attached to the boney skeleton with threaded and/or smooth pins and/or threaded and/or smooth and/or beaded wires. Such constructs are commonly referred to as orthopaedic external fixators or external skeletal fixators. External fixators may be utilized to treat acute fractures of the skeleton, soft tissue injuries, delayed union of the skeleton when bones are slow to heal, nonunion of the skeleton when bones have not healed, malunion whereby broken or fractured bones have healed in a malposition, congenital deformities whereby bones develop a malposition, and bone lengthening, widening, or twisting.
External fixators vary considerably in design and capabilities, and may include multiple or single bars or rods, and a plurality of clamps for adjustably securing the bars to pins or wires which are, in turn, joined to the boney skeleton. The pins or wires may extend completely through the boney skeleton extending out each side of the limb or may extend through the boney skeleton and out one side of the limb. Pins which extend completely through the boney skeleton and out both sides of the limb are commonly referred to as "transfixation pins." Pins which extend through the boney skeleton and out only one side of the limb are commonly referred to as "half pins." Such external fixators may be circumferential for encircling a patient's body member (e.g., a patient's femur), or may be unilateral for extending along one side of a patient's body member. More that one unilateral external fixator can be applied to the same length of the patient's body member. Materials for fixators also vary, including metals, alloys, plastics, composites, and ceramics. External fixators vary in their ability to accommodate different spatial relations between the pin and bar.
Prior art external fixators stabilize bone fragments by holding the fragments in a relatively fixed spatial relation. Some of the more completely adjustable external fixators allow the physician to reorient one fragment with respect to the other along all six axes in an acute motion, usually by loosening one or more clamps and effecting the corrective motion manually and retightening clamps to hold the fragments stably.
A circumferential external fixator system was disclosed by G. A. Ilizarov during the early 1950's. The Ilizarov system include at least two rings or "halos" that encircle a patient's body member (e.g., a patient's leg), connecting rods extending between the two rings, transfixion pins that extend through the patient's boney structure, and connectors for connecting the transfixion pins to the rings. Use of the Ilizarov system to deal with angulation, translation and rotation is disclosed in "Basic Ilizarov Techniques," Techniques in Orthopaedics.RTM., Vol. 5, No. 4, December 1990, pages 55-59.
Mears, U.S. Pat. No. 4,620,533, issued Nov. 4, 1986, discloses a unilateral external fixator system including a plurality of fixation pins attached to at least one rigid bar through adjustable clamps having articulating balls which allow rotational adjustment of each pin or bar.
Stef, U.S. Pat. No. 5,209,750, issued May 11, 1993, discloses a unilateral external fixator system including an orthopaedic brace for rigidly connecting groups of pins screwed into a long bone for the reduction of a fracture of the long bone. The brace includes a telescopic support made up of an elongated tube and an elongated rod slidable within the tube. A first plate is attached to the outer end of the tube and a second plate is attached to the outer end of the rod. Third and fourth plates are adjustably attached to the first and second plates, respectively, by way of threaded rods and ball-and-socket joints. Jaws are attached to each third and fourth plate to secure the pins to the brace.
Prior art orthopaedic external fixators differ in their ability to move or adjust one bone fragment with respect to the other in a gradual fashion. Some allow gradual translation, others allow gradual rotation about two axes. The Ilizarov system can provide an external fixation device that could provide gradual correction along and about six axes; however such a device would require many parts and would be relatively complicated to build and use in a clinical situation.
Often orthopaedic external fixators such as Ilizarov fixators must be modified later on after their initial application. Such modification may be necessary to convert from one correctional axis to another or to convert from an initial adjustment type of fixator to a weight bearing type of fixator, some of the correctional configurations not being stable enough for weight bearing.
More simplistic external fixators may accomplish a rotation of fragments about a center of rotation contained on the external fixator. This may or may not correspond to the center of rotation necessary to fully correct the deformity by angular correction alone. In no circumstances will a center of rotation confined to the external fixator create a virtual center of rotation remote to the external fixator as is frequently required in the treatment of these deformities. Some orthopaedic external fixators utilize a simple hinge which cannot create a center of rotation remote to its mechanism. The Ilizarov system provides a circumferential encompassing type fixator that is more universal in that it permits the placement of the hinge axis around the bone, but does not allow rotation about an axis remote to its mechanism. A focal hinge made of an arc segment of gear or track with a following carriage can create a center of rotation remote to the mechanism but may not be applicable to certain situations where because of anatomy or preference the mechanism is to be applied to the concavity of a deformity, especially a severe deformity where there is no space to apply the long arc segment of gear or track necessary to fully correct the deformity.
Anderson, U.S. Pat. No. 2,391,537, issued Dec. 25, 1945, discloses an orthopaedic external fixator for fracture reduction including a pair of hollow tubes telescopically joined together, a plurality of pins for transfixing bone elements, a first fixation unit slidably mounted on one of the tubes for connecting a pair of the transfixion pins to that tube, and a second fixation unit attached to the end of the other tube for connecting a pair of the transfixation pins to that tube. One of the tubes is telescopically mounted within the other tube. A threaded adjusting shaft is mounted within the tubes and can be manually rotated by way of a wrench head located at the outer end of one of the tubes. Rotation of the shaft causes a nut nonrotatably located within the tubes to move longitudinally along the shaft. Coil springs located within the tubes on either side of the nut transfer longitudinal movement of the nut to the tubes while permitting a certain desired yielding and eliminating any perfectly solid and hard contact. A geared mechanism allows for correction of rotational deformity, utilizing an arc segment of gear and a mating carriage with corresponding pinion.
A "Stewart platform" is a fully parallel mechanism used in flight and automotive simulators, robotic end-effectors, and other applications requiring spatial mechanisms with high structural stiffness; and includes a base platform, a top platform, and six variable limbs extending between the base and top platforms. See S. V. Sreenivasan et al., "Closed-Form Direct Displacement Analysis of a 6-6 Stewart Platform," Mech. Mach. Theory, Vol. 29, No. 6, pp. 855-864, 1994.
Nothing in the known prior art discloses or suggests the present invention. For example, nothing in the known prior art discloses a fixator that can be adjusted in six axes by changing strut lengths only, without requiring joints to be unclamped, etc. Further nothing in the known prior art discloses or suggests a mechanism including, in general, a first member or swash plate for attachment relative to a first element; a second member or swash plate for attachment relative to a second element; an adjustable effective length first strut having a first end movably attached to the first member and a second end movably attached to the second member; an adjustable effective length second strut having a first end movably attached to the first member and a second end movably attached to the second member; an adjustable effective length third strut having a first end movably attached to the first member and a second end movably attached to the second member; an adjustable effective length fourth strut having a first end movably attached to the first member and a second end movably attached to the second member; an adjustable effective length fifth strut having a first end movably attached to the first member and a second end movably attached to the second member; and an adjustable effective length sixth strut having a first end movably attached to the first member and a second end movably attached to the second member, with the first ends of the first and second struts joined relative to one another so that movement of the first end of one of the first and second struts will cause a corresponding movement of the first end of the other strut, with the first ends of the third and fourth struts joined relative to one another so that movement of the first end of one of the third and fourth struts will cause a corresponding movement of the first end of the other strut, with the first ends of the fifth and sixth struts joined relative to one another so that movement of the first end of one of the fifth and sixth struts will cause a corresponding movement of the first end of the other strut, with the second ends of the first and sixth struts joined relative to one another so that movement of the second end of one of the first and sixth struts will cause a corresponding movement of the second end of the other strut, with the second ends of the second and third struts joined relative to one another so that movement of the second end of one of the second and third struts will cause a corresponding movement of the second end of the other strut, with the second ends of the fourth and fifth struts joined relative to one another so that movement of the second end of one of the fourth and fifth struts will cause a corresponding movement of the second end of the other strut.